Refactor to use builder pattern for Event payload models.

[mikecdavis]

There were a lot of constructor arguments for the payload models that made reviewing the EventBuilder code difficult to parse. I've converted them all over to builders to help with readability and maintainability going forward.

I also renamed EventBuilder to EventFactory, I think that is more semantically correct (at least to me). Unfortunately, in the overall diff github isn't rendering it as a rename. If you look at the individual commits it does.

[coveralls]

Pull Request Test Coverage Report for Build 563

• 197 of 201 (98.01%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.8%) to 89.399%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-api/src/main/java/com/optimizely/ab/event/internal/payload/Event.java	23	25	92.0%
core-api/src/main/java/com/optimizely/ab/event/internal/payload/Visitor.java	11	13	84.62%

Totals
Change from base Build 551:	5.8%
Covered Lines:	2336
Relevant Lines:	2613

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💛 - Coveralls

[mikeproeng37]

lgtm, thanks for the refactor!